It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone", the presence of the marker is detected and an alarm is generated.
One widely used type of EAS system employs magnetomechanical markers that include a magnetostrictive element. An example of a marker of this type is illustrated in schematic, exploded form in FIG. 1. Reference numeral 10 generally refers to the magnetomechanical marker of FIG. 1. As seen from FIG. 1, components of the marker 10 include a magnetostrictive active element 12 and a housing 14 which forms a recess or cavity 16 in which the active element 12 is placed. A bias or control element 18 is fixedly mounted to the housing 14 adjacent to the active element 12. An adhesive layer (not shown) may be applied to the bottom or top of the housing to secure the housing to an article of merchandise.
Both the active and bias elements typically are in the form of ribbon-shaped lengths of metal alloy. Known active elements are cut from melt-spun amorphous alloy ribbons, and exhibit soft magnetic properties and substantial magnetostriction. Bias elements should exhibit hard or semi-hard ferromagnetic properties.
The active element is fabricated such that it is mechanically resonant at a predetermined frequency when the bias element has been magnetized to a certain level. At the interrogation zone, a suitable oscillator provides an AC magnetic field at the predetermined frequency, and the magnetostrictive element mechanically resonates at this frequency upon exposure to the field when the bias element has been magnetized to the aforementioned level. The resulting signal radiated by the magnetostrictive element is detected by detecting circuitry provided at the interrogation zone.
A magnetomechanical marker of the type illustrated in FIG. 1, and an EAS system which operates with this type of marker, are disclosed in U.S. Pat. No. 4,510,489, issued to Anderson et al. The disclosure of the Anderson et al. patent is incorporated herein by reference.
EAS systems which use magnetomechanical markers have proved to be very effective. Systems of this type are sold by the assignee of this application under the brand name "ULTRA*MAX".
The Anderson et al. patent points out the need to form the housing for the marker so that the mechanical resonance of the active element is not mechanically damped. That patent also teaches that the marker should be formed so that the bias element does not mechanically interfere with the vibration of the active element.
Although it is necessary to provide some freedom of movement for the active element, this requirement can lead to problems in operation of EAS systems which use magnetomechanical markers. One problem arises from the fact that the resonant frequency of the active element tends to be somewhat sensitive to variations in the bias field applied to the active element. If the active element shifts relative to the bias element, particularly in the longitudinal direction of the element, there may be a change in the resonant frequency of the active element so that the resonant frequency departs from a nominal frequency at which the marker is to be detected. As a result, detection of the marker may be compromised.
In addition, the marker, in actual use, may be placed so that the long dimension of the housing is vertically oriented. As a result, the lower end of the active element is likely to come into contact with a side of the housing, resulting in a mechanical loading of the lower end of the active element. This loading of the end of the active element may tend to cause frictional damping of the mechanical resonance of the active element, reducing the amplitude of the signal generated by the active element and potentially interfering with detection of the marker.